Another name for sweet potatos are yams. They range in color from yellow to deep orange. Depending on where you live, they may or may not be available year round. Typically in the fall they are available.
A rule of thumb that I've heard, although it's very hard for me to follow, since we live up north, is to eat 5 fist-sized portions of vegetable every day, in at least 3 different colors (like green, orange, red for example).
P.S. Other good substitute for sweet potato is yellow or orange winter squash (butternut, etc.). High in beta carotene, etc.